Closely Packed Stretchable Ultrasound Array Fabricated with Surface Charge Engineering for Contactless Gesture and Materials Detection

Abstract Communication with hand gestures plays a significant role in human‐computer interaction by providing an intuitive and natural way for humans to communicate with machines. Ultrasound‐based devices have shown promising results in contactless hand gesture recognition without requiring physical contact. However, it is challenging to fabricate a densely packed wearable ultrasound array. Here, a stretchable ultrasound array is demonstrated with closely packed transducer elements fabricated using surface charge engineering between pre‐charged 1–3 Lead Zirconate Titanate (PZT) composite and thin polyimide film without using a microscope. The array exhibits excellent ultrasound properties with a wide bandwidth (≈57.1%) and high electromechanical coefficient (≈0.75). The ultrasound array can decipher gestures up to 10 cm in distance by using a contactless triboelectric module and identify materials from the time constant of the exponentially decaying impedance based on their triboelectric properties by utilizing the electrostatic induction phase. The newly proposed metric of the areal‐time constant is material‐specific and decreases monotonically from a highly positive human body (1.13 m2 s) to negatively charged polydimethylsiloxane (PDMS) (0.02 m2 s) in the triboelectric series. The capability of the closely packed ultrasound array to detect material along with hand gesture interpretation provides an additional dimension in the next‐generation human‐robot interaction.


Supplementary Note (Note S1): Variation of the time constant due to changes in orientation
The fringe capacitance and fringe electric field depend on the orientation (concave or convex) along with the area of the electrodes (Fig S18).The COMSOL electrostatics simulation shows that the slope of the relative change in fringe capacitance due to the relative change in geometric area is approximately 22% higher for convex orientation than concave orientation (the same magnitude of the curvature for both orientations).Therefore, the effective area projected by the electrostatic field lines for convex orientation is approximately 22% higher than the concave orientation for the same geometric area (∆ ∝ ∆ ).Here, the material-specific areal-time constant S depends on the material and remains the same, so the time constant  lowers for convex orientation as  = 1/ remains a constant.This is consistent with the experimental result that the time decay constant increases as the hand switches from the convex to concave orientation (Fig. 5E).It is important to note that only the polarity is changed to result in a change in orientation (convex and concave) in the simulation but the magnitude of curvature is kept the same.
The changes in the magnitude of curvature would change the distance, dielectric material, and the slope of relative change in fringe capacitance due to relative change in the geometric area.

Supplementary Note (Note S2): Non-contact Triboelectric Sensor
Triboelectricity can be generated by utilizing the surface charge of materials.It can be produced through both contact and non-contact electrification.Contact triboelectricity involves direct contact and separation of two different materials (according to triboelectric series), leading to electron transfer and energy generation.Devices utilizing contact triboelectricity, such as triboelectric generators, come in physical contact to produce electric charges.On the other hand, non-contact triboelectricity operates without direct physical contact, relying on electrostatically induced surface charges from external stimuli such as vibrations or movements.Compared to contact triboelectric systems, the non-contact method offers higher durability and flexibility in applications such as touchless interfaces and energy harvesting from ambient sources.While contact triboelectricity may require maintenance due to wear, non-contact triboelectric systems are often considered more environmentally friendly due to high durability and energy harvesting from ambient movements.Capacitive sensors operate on the principle of detecting changes in capacitance, thereby changing the impedance as well.Capacitive sensors are widely used in touchscreens, proximity detection systems, and various applications where non-contact sensing is essential.Triboelectric and capacitive sensors fundamentally follow Maxwell's Laws, so it is more about the current terminology used in the literature that differentiates them, especially noncontact triboelectric and capacitive sensors.Here, charge decay in the static phase of the sensor is one of its essential aspects.Characterization of surface charge decay is not of utmost importance and, therefore, not well discussed in the capacitance sensor terminology, whereas in triboelectricity, there are several literature reports discussing increasing or decreasing charge decay 1 .

Fig S2 :
Fig S2: Optical image of the stretchable ultrasound array with closely packed transducer elements.

gFig S5 :Fig S6 :
Fig S5: Optical images demonstrating the attachment of the PI film with various thicknesses on the PZT transducer element.

Fig S8 :
Fig S8: Invariance in the impedance or current density during the motion of the object from the middle (top) to the right (bottom) rules out the time-of-flight mechanism.

Fig S12 :
Fig S12:The cross-sectional view of each ultrasound element in the stretchable array.

Fig S13 :Fig S14 :
Fig S13: Yield stress and plastic strain of Cu/PI interconnects.The Young's modulus of Cu (or PI) is 90 GPa (or 2.5 GPa) and the Poisson ratio of Cu and PI is 0.34.

Fig S15 :
Fig S15: The orientation of different transducer elements in the array used for COMSOL simulation.

Fig S16 :
Fig S16: Temporal variation in the impedance during the vertical movement of one finger at varying distances: 1, 2, and 4 cm.

Fig S17 :Fig S18 :
Fig S17: Invariance of triboelectricity during the change of the orientation or curvature (negative or positive) of the hand.

Fig S19 :
Fig S19: Invariance of the time decay constant during random surface contamination